In the case of conventional drive trains in motor vehicles, differences in engine rotational speed during driveaway and when shifting gears is compensated for by friction clutches, respectively variable speed gears. Hybrid drives expand the conventional drive train by an electric machine, which can be connected to the internal combustion engine via an additionally shiftable clutch, particularly a so-called starter clutch.
Automated, electrohydraulic clutches are frequently used as a starter clutch. A suitable operating strategy controls the additional starter clutch in a hybrid drive such that purely electrical driving and braking is possible in selected driving situations. The additional starter clutch is also used in order to restart the internal combustion engine when necessary. The starter clutch is thereby so embodied that it is closed (normally closed) in the event of a malfunction (pressureless). In this way, conventional driving is still possible in the event of malfunctions, such as a leak or a hardware defect of the actuator of the said starter clutch.
It is advantageous for the energy recovery during overrun conditions (recuperation) to decouple the drag torques of the internal combustion engine. For that reason, the starter clutch is opened and held open in this vehicle state. As a function of the operating strategy which is used, the hold-open time in the New European Driving Cycle (NEDC) can be up to 50%. The starter clutch must therefore be held open against a closing spring force for half of the operating time. Known starter clutches thereby require a power consumption of approximately 30 W to hold the starter clutch open. 30 W must therefore be constantly applied during 50% of the operating time in order to hold said starter clutch open. An additional, not inconsiderable fuel consumption results therefrom.